As one who had to deal with the children who were sent to school when ill, I offer another perspective.
Little Felicity, sent in to school with "a bit of a temperature, but I've given her some Calpol," suffers projectile vomiting an hour later during a maths lesson, spraying the contents of her stomach over several classmates and the floor.
Maths lesson is abandoned as my classroom assistant and I organise the cleaning up and try to stop the domino affect that the sight and smell of vomit has on small children.
Two children start retching so I usher 30 children outside for a quick PE session and try to include a bit of maths at the same time.
The following day the bug spreads through my class and I keep plastic bowls dotted around the room, "just in case."
The whole school gets affected in the end and staff start dropping like flies.
Best of all some parents blame the school for the situation and, thanks to the playground mafia, we get a series of irate letters dropped of.
Just one example. I could give you more. Remind me to tell you the story of the girl with the broken leg who needed escorting to the toilet at regular intervals, or best of all, the lady who gave her son a 5ml spoonful of Ibobrufen gel, told us what she'd done, was horrified when I told her that it was for external use only then cleared off to work and left me to deal with the aftermath!